This invention relates generally to a feedback scheme for an electronic ballast and, more particularly, to a scheme for minimizing overboost voltage conditions due to feedback.
Conventional ballasts, such as disclosed in U.S. Pat. No. 5,404,082, are smaller and less costly, in part, through elimination of a preconditioner stage. Such ballasts also include an output stage formed from an inverter and a resonant circuit and are commonly referred to as a single stage inverter.
The preconditioner stage following lamp ignition (i.e. during steady state conditions) is often used to increase (i.e. boost) the ballast input voltage in conditioning the voltage applied to the inverter. In the absence of the preconditioner stage, boosting of the ballast input voltage is achieved by supplementing the ballast input voltage with a high frequency signal fedback from the resonant circuit. This high frequency signal represents the resonant inductor current.
The ballast input voltage (typically rectified) and then boosted is applied to a buffer capacitor which serves as a DC source for the inverter. When the high frequency feedback signal is higher than necessary, an overboost voltage is impressed across the buffer capacitor. Under overboost voltage conditions, a very high stress is placed on various ballast components including the buffer capacitor and switches within the inverter. The buffer capacitor is typically of the electrolytic type. The inverter switches are typically power MOSFETs. Both the buffer capacitor and inverter switches can be damaged and fail under overboost voltage conditions.
During lamp ignition, ballast schemes such as disclosed in U.S. Pat. No. 4,511,823, also lead to overboost voltage conditions and are due, in part, to current flowing through the resonant inductor being fedback and charging the buffer capacitor. Complicated control circuitry is required to avoid such overboost resulting in a more costly and more difficult ballast to manufacture.
It is therefore desirable to provide an improved ballast feedback scheme which reduces the high frequency feedback signal in order to avoid overboost voltage conditions during ignition and steady state operation. The scheme should be particularly applicable to an inverter having a single stage with current feedback and high power factor.